13-3 The Galilean satellites formed like a solar system in miniature

As Table 13-1 shows, the more distant a Galilean satellite is from Jupiter, the greater the proportion of ice within the satellite and the lower its average density. But why should this be? An important clue is that the average densities of the planets show a similar trend: Moving outward from the Sun, the average density of the inner six planets steadily decreases, from more than 5000 kg/m³ for Mercury to less than 1000 kg/m³ for Saturn (recall Table 7-1). The best explanation for this similarity is that the Galilean satellites formed around Jupiter in much the same way that the planets formed around the Sun, although on a much smaller scale.

We saw in Section 8-5 that dust grains in the solar nebula played an important role in planet formation. In the inner parts of the nebula, close to the hot protosun, only dense rock and metal grains were able to remain solid. These grains accumulated over time into the dense, rocky inner planets. But in the cold outer reaches of the nebula (beyond the “snow line”), dust grains were able to retain icy coatings of low-density material like water and ammonia. Hence, when the Jovian planets formed in the outer solar nebula and incorporated these ice-coated grains, the planets ended up with both rock and ices in their cores as well as substantial amounts of ammonia and water throughout their interiors and atmospheres.

As Jupiter coalesced, the gas accumulating around it formed a rotating “Jovian nebula.” The central part of this nebula became the huge envelope of hydrogen and helium that makes up most of Jupiter’s bulk. But in the outer parts of this nebula, dust grains could have accreted to form small solid bodies. These grew to become the Galilean satellites.

Although the “Jovian nebula” was far from the protosun, not all of it was cold. Jupiter, like the protosun, must have emitted substantial amounts of radiation due to Kelvin-Helmholtz contraction, which we discussed in Section 8-4. (We saw in Section 12-4 that even today, Jupiter emits more energy due to its internal heat than it absorbs from sunlight.) Hence, temperatures very close to Jupiter must have been substantially higher than at locations farther from the planet. Calculations show that only rocky material would condense at the orbital distances of Io and Europa, but frozen water could be retained and incorporated into satellites at the distances of Ganymede and Callisto. In this way Jupiter ended up with two classes of Galilean satellites: primarily rocky satellites versus satellites made of mixture of rock and water (Figure 13-3).

The analogy between the solar nebula and the “Jovian nebula” is not exact. Unlike the Sun, Jupiter can be thought of as a “failed star.” Its internal temperatures and pressures never became high enough to ignite nuclear reactions that convert hydrogen into helium. (Jupiter’s mass would have had to be about 80 times larger for these reactions to have begun.) Furthermore, the icy worlds that formed in the outer region of the “Jovian nebula”—namely, Ganymede and Callisto—were of relatively small mass. Hence, they were unable to attract gas from the nebula and become Jovian planets in their own right. Nonetheless, it is remarkable how the main processes of planet formation seem to have occurred twice in our solar system—once in the solar nebula around the Sun, and once in microcosm in the gas cloud around Jupiter.

The diverse densities of the Galilean satellites suggest that these four worlds may be equally diverse in their geology. This conclusion turns out to be entirely correct. Because each of the Galilean satellites is unique, we devote the next several sections to each satellite in turn.